1. Field of the Invention
This invention relates to a high strength steel sheet which is resistant to rupture or generation of cracks at sheet end surfaces during hole expansion by punching or the like. Such a steel sheet is referred to herein as one having excellent stretch flanging formability.
2. Description of the Related Art
Nowadays, a weight reduction by strengthening is an important characteristic of steel sheets intended to be exposed to working.
To strengthen a steel sheet intended for working, a 2nd-phase strengthening method is generally employed which utilizes the so-called 2nd phase of the steel sheet. Such a 2nd-phase-strengthened steel excels not only in balance between strength and ductility but also in such properties as yield ratio (YR=YS/TS) and long life, where YR means yield ratio, YS means yield strength and TS means tensile strength.
A problem with such conventional 2nd-phase-strengthened steels is that when they are subjected to press working involving stretch flanging, as in the case of hole expansion, they are subject to rupture due to cracks generated in their end surfaces because they do not have sufficient stretch flanging formability.
As a means for overcoming the problem a method has been proposed in Japanese Patent Laid-Open No. 61-48520, comprising a combination of reduction in the 2nd phase, minute distribution thereof, improvement in surface properties, etc. However, such a combination of optimized factors only results in complication of the process control procedures. Moreover, it does not help to prevent distortion from being introduced into the 2nd-phase, which distortion constitutes a deteriorating factor of stretch flanging formability. Thus, no great improvement could be expected from the proposed method.